In connection with automotive vehicles, especially passenger cars, constant speed control devices (also called cruise control devices) are known for automatic control of the vehicle's speed in relation to a pre-set target value. Such cruise control functions will normally be included as a function of a computerized control unit in the vehicle. A setting device, by which the driver can set a target value corresponding to the desired vehicle speed, is connected to the control unit. The control unit is adapted to control the operation of the vehicle's engine, dependent upon the currently set target value, in such a manner that the vehicle will reach the speed corresponding to the target value. This control is performed in dependance of various operating parameters of the vehicle, such as, for example, the current engine load.
The control towards the set target value is normally performed by comparison of a signal corresponding to the target value with an actual value; that is, a current value of the vehicle's speed, whereby the latter value can be determined by an existing speedometer provided in the vehicle. Depending on the difference existing between the actual value and the target value at the time of measurement, the vehicle fueling throttle can be set to a certain angular position. This angular position in turn corresponds to a certain amount of air supplied to, and thus to a certain output torque from, the engine. In this manner, the desired target value regarding vehicle speed can be reached. In such a manner the throttle can be controlled mechanically, or may alternatively consist of an electrically controlled throttle, also called an "E throttle." In the latter case, the throttle angle is adjusted by a servomotor connected to the control unit and coupled to the throttle.
Cruise control may also be performed by means of a control method utilizing what is referred to as a P controller; that is a controller based on proportional control. This type of controller multiplies, in the control unit, the difference between the present actual value and the target value by a predetermined gain factor. This provides an output value that is subsequently used by the control unit for adjusting the throttle.
The P control method discussed above may advantageously be complemented by an integrating factor; i.e., so as to obtain a PI controller. An output value is calculated, in the control unit, which consists of the sum of a proportional portion (a "P portion") and an integrated portion (an "I portion") in which the latter portion is based on an integration of the difference between the vehicle velocity and the target value, over time. This total output value is then utilized for adjusting the throttle angle to a desired value.
When cruise control is used in a vehicle, a driving situation may sometimes occur demanding a torque increase from the engine, which in turn corresponds to a demand for an increased airflow to the engine. Such a situation might for example occur if the vehicle, after being driven on level ground with a moderate load and at constant speed, reaches a steep uphill slope. With a cruise control based on previously known PI regulators, in such a situation, the proportional portion, as well as the integrated portion will increase successively at this stage, as the current actual value will fall below the target value as the vehicle reaches such an uphill slope. An increase of the proportional and the integrated portions will of course also lead to an increase of the sum of these two control factors. According to known techniques, an increase of this sum corresponds to an increase of the throttle angle. This entails a successive opening of the throttle until the vehicle speed reaches the set target value again. In this context, however, it should be noted that in today's vehicles there is normally a relationship between the set throttle angle and the amount of air supplied, which entails that after having increased the throttle angle up to a certain value (for example in connection with the situation described above), any further increase of this angle will not provide any further substantial increase in the amount of air fed into the engine. This also means that an increased angle will not provide any substantial increase in the engine's output torque. Vice versa, the above relationship entails, that when the integrated portion (and consequently also the throttle angle) decreases from a state of very large throttle angle, there will initially not be any substantial decrease in the amount of air and thus neither any substantial decrease of the engine torque.
The above situation may then lead to the following problem: If the vehicle, after being driven at a high torque, reaches a condition requiring a relatively low torque, (which could be said to correspond to the vehicle reaching the crest of the hill after being driven uphill for a distance during which the throttle angle would be relatively large) there will continue to be a high torque output from the engine causing the vehicle to accelerate rapidly. This is registered by the control unit, as the actual value then exceeds the target value, and will in turn lead to a decrease of the integrated portion, as well as the proportional portion of the value calculated by the PI controller. This will also cause the throttle angle to decrease. Accordingly, this decrease will, however, entail only a very slow decrease of the supplied amount of air, and thus torque, as the integrated portion was previously allowed to rise to a very high value during the demand for increased engine torque. As a consequence of the relationship existing between the set throttle angle and the amount of air fed to the engine, the engine output torque will thus be generally constant for a relatively long period of time, while the integrated portion decreases. This causes the vehicle to accelerate rapidly after having reached the crest of the hill, which is, of course, a disadvantage.
According to known techniques, the above problem can be solved at least in part by arranging the control unit to limit the integrated portion to a maximum allowed value.
In view of the above described deficiencies associated with conventionally designed methods and devices for providing cruise control in a vehicle, the present invention has been developed. These enhancements and benefits are described in greater detail hereinbelow with respect to several alternative embodiments of the present invention.